Bifocal-lens blank



Patented Wiley ac, ieaa.

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GEORGE A. CLEMENT, 0F MOUNT VERNON, NEW YORK, ASSIGFlOR T0 GENERAL OPTI-CAL COMPANY, A CORPORATION OF NEW YGRK. Y

' BIFOCAL-LENS BLANK.

Application filed April 9, i923. Serial No. 630,880.

T 0 all whom it may cmwern: Be it known that I, GEORGE A. CLEMENT, acitizen of the United States, residing at Mount Vernon, in the county ofWestchesler and State of New York, have invented certain new and usefulImprovements in Bifocal-Lens Blanks, of which the following is a full,clear, and exact description.

In the manufacture of blanks from which fused bifocal lensesar'eproduced, the practice is to grind and polish a recess or depressionin one'face of a major portion or n'piece of glass near one edgethereof, and

seat therein a smaller piece of glass, known in the trade as a button,having its under surface properly shaped and repared to unite with thatof the recess an index of refractiongreater than that of the majorpiece. In meniscus lenses the recess or depression is in the outer orconvex face. The two pieces of glass are then subjected to heat untilthey unite by fusion forming one integral lens blank, and the finishedlens is made by grinding and polishing the'two faces of the blank to theproper curvatures according to the desired lenticular and cylindricalpowers. While the curvature of the bifocal side is uniform throughoutits entire-area, the focal distance through the segment or area occupiedby the button, which constitutes the reading segment or field, isshorter than that through the rest of the lens which constitutes thedistance field, because of the difference in refractive index throughthe two fields. This result is obtained, for example, by making themajor portion of crown glass and the buttonof flint glass.

Since the power of a lens depends upon the curvature of its faces andits index of refraction, it follows that the addedpower' through thereading field depends both upon the curvature ofthe recess ordepression, i. e.', .the curvature on the inner face of the button, andthe curvature of the outer or bifocal face of the major lens, whichcurvature is also common to the reading segment.

According to present standardpractice the curvature ofthe depressionvaries in' a complete stock of fused bifocal lens blanks from a maximumconcavitv for the highest addition to a plane or substantially planesurface, the refractive index of the flint glass buttons being constantthroughout the enhaving an tire range. In rare cases where the addedpower called for by a prescription is very ugh the manufacturer may makea special air of blanks using a flint glass of higher index ofrefraction in order to reduce the concavity and hence the thicknessofthe toric blanks are put out in a number of so-. I

called base curves in order to minimize the work of the lensgrinder inreducing the surfaces of the blanks to the desired curvatures. The outeror convex face of the blank may be finished to any curvature within therange of the tools. of the lens grinder, and the lens grinder willselect a blank having a base curve nearest to that of the finishedsurface.

reading field will depend upon the final curve of the convex face, sincethis determines the outside curve of the minor lens. Therefore in orderto obtain a desired addi- Itis manifest, however,that with a given blankthe added power through the,

t'ion, as this added power is commonly' of satisfying. the ordinaryprescriptions.-

Briefl-y' stated-it contemplates a set of fused bifocal lens blanks inwhich the geometrical surface of the recess is constant. This has manyadvantages, some of which will pres ently be pointed out, and I havedetermined by calculation and test that 'the refractive index of theflint glass used for the buttons may be so varied as to provide therequisite range of additions without modifying the inside-curve of thebuttons, and at the same time permitting the grinder to use the presentstandard surfacingtools.

By way of illustration I shall explain my invention as it will beemployed so that the two surfaces which are united by fusing are plano.The advantages in fusing two plano surfaces together as compared withthe fusing of two spherical surfaces will be appreciated by thoseskilled in the art. In the present practice in which the contactlngsurface of the button is convex, it is ground to a greater curve than isthe recess and is seated in the recess and held in contact at one pointat the initiation of the fusing operation. The button is then heateduntil it gradually drops down from the point of contact outwardly andcoalesces with the wall of the depression, this method bein employed toexclude all air and evolve gases from between the two surfaces. For thispurpose the fusing temperature required is so great that the glass atthe surfaces is caused to deteriorate and the contact surfaces arefrequently distorted. This is entirely eliminated in the fusing of twoplano surfaces together since a considerably lower temperature isrequired therefor and it is unnecessary to heat the glass to such. adegree as to affect it deleteriously.

Another decided advantage is a set of blanks in which the countersink orrecess is practically eliminated, irrespective of the addition. is thefact that the finished lenses may be thinner and correspondinglylighter, since they enable the lens grinder to grind the concave face oflenses of hi h addition as far down as that of lenses 0 the lowestaddition.

Another feature of my invention is the fact that it reduces thechromatic aberration of the reading field for the higher additions ascompared with the present type in which flint glass of the same index ofrefraction and with its relatively high dispersion is used for both thehigh and low additions.

Still another advantage of the set of blanks comprehended by myinvention is the fact that the cost of manufacture is materiallyreduced. For example with the plane interfaces the crown discs may bemuch more expeditiously prepared to receix e the buttons than is thecase where the countersinks are spherical; they are more easily blockedup and many more may be operated on at one time. And in any event it isevident that a considerable saving in time and grinding apparatusis-effected by having all of the interfaces the samegeometricalsurfaceyand that economy results from the saving of time andheat required for the fusing operation, and from the substantialreduction in the number of defective blanks.

Many other advantages in my invention than those above pointed out willappear to those skilled in the art. I shall now explain one way in whichmy invention may be reduced to practice and will thereafter point out myinvention in claims.

Figure 1 is a sectional elevation of the two parts of a blank assembledready for fusing;

Figure 2 is a similar view of the blank after fusing;

Figure 3 is a plan of the blank after its bifocal side has been surfaceddown to the desired curvature, as indicated by the dotted line in Figure2;

Figure 4 is a table of seven different flint glasses which may beemployed in carrying out the invention, giving their indices of refraction on the D line together with the surface curvatures of lensesmade from them which will be necessary to effect an addition of onediopter;

Figure 5 is a chart illustrative of the use of the seven flint glassesin conjunction with a major lens of crown glass of refractive index1.523, and showing the numbers of the flint glasses and the surfacecurvatures which will provide the customary additions; an

Figure 6 is a chart similar to that of F igure 5 except that it islimited to three different flint glasses of the seven tabulated inFigure 4.

Figures 1, 2 and 3 show a rough major blank 1 of crown glass'and abutton 2 of flintg-lass. A small portion 3 of the convex face of themajor blank has been ground and polished off to a plane surface. Thebutton 2 is a plane disc, the under face of which is ground and polishedto a plane surface. Preferably the two parts will be placed in a furnacein the relation illustrated in Figure 1, with a small insert or wedge 4underneath the lower edge of the button so as to hold it in tiltedrelation with its upper edge resting at one point. upon the surface ofthe crown glass blank. Under the influence of the heat the flint glassbutton becomes plastic and drops down so as to conform to and coalescewith thesurface of the crown glass. as shown in Figure 2. In producingthe finishedlens from this fused blank. the top surface is ground off tothe desired curvature so as to include a suflicient portion of the flintglass button, for example as illustrated by the dotted line in Figure 2.The under face of the blank is then ground according to prescription.

and the finished lens is cut out to the de-' sired shape. for instanceas indicated in the dotted outline in "Figure 3. lit is evident that theadded power through the reading segment'depends upon the index ofrefraction on the flint glass and upon the. curvature of the outer faceof the lens. since the lower face of the button is plano.

l have determined that if the seven flint glasses tabulated in Figure 4are ground plano-convex with the convex curvatures in- I 'dicated in thethird column of Figure 4. they will all add one diopter in lenticularaeeaese the convex face of the blank be ground to six" diopters, theadded power will be .50 diopters, and. other additions-may be similarlycomputed;

From the table of, Figure 4 it follows that if the button 2 be made of#2 flint glass having an index of refraction of 1.5753, thev additionwill be'one diopter if the curvature of the outer face of the blank beten diopters. If the button be made of #3 flint glass having an index ofrefraction 1.5884,

" two diopters.

the addition will be one diopter if the outer face be ground to eightdiopters. If the button be #4 flint glass with an index of refraction of1.6002, the addition will be one diopter if the curvature on the outerface be six diopters. If the button be made of #5 flint glass having anindex of refraction of 1.6537, the addition will be one diopter if thecurvature of the'outer face of the blank be four diopters. If the buttonbe made of #6 flint glass having an index of refraction 1.6973, theaddition will be one diopter if. the curvature of the outer face be.three diopters. If the button be composed of #7 flint glass having anindex of refraction 1.7 845, the addition will be one diopter if thecurvature of the outer face of the blank be In the chart of Figure 5there is indithe number of flint glass which would serve to produceadditions from .25 to 4.50. It will be understood that all the possiblecombinations are nottherein illustrated, but only such as might easilyand reasonably be employed. -In the first column is shown onecombination which will produce an addition of .25 diopters. This is #1flint glass with acurvature of three diopters, F standing for flintglass and C standing for curvature.

If #1 flint glass adds one diopter with the curvature on the outer faceof the blank of twelve diopters, it therefore follows that it will'add.25 diopters if the outer curvature is three diopters. In the secondcolumn;

there are shown four combinationsof flint glass and surface curvatureswhich will produce an addition of .50 diopters.- These are the firstfour of the seven flint glasses with the curvatures just half of thatindicated in thethird column of Figure 4. In the third column of Figure5 there are shown four combinations which will produce an addition of.75, these being the first four numbars of flint glass with thecurvatures'threequarters of the respective numbers appearing in thethird column of Figure 4; and so it will be seen that the successivecolumns are simply the result of mathematical computation until thehighest addition, 4.50 diopters, is reached. It will be seen that thecurvature of #7 flint glass in order to produce this addition will needto have 4%;

times the curvature which is set opposite it in Figure 4 i. e. acurvature of nine di-' opters. Within this range of additions areincluded all of the ordinary prescriptions.

These blanks could be made on any base curve plan and are readilyadaptable to the 30 present three base curve plan. viz, three diopters,six-diopters and nine diopters. This would give a. maximum of twenty onenumbers of blanks to satisfy all prescriptions,

i. e., seven difi'erent numbers of flint glass of each of the basecurves. As will readily be. seen from van inspection of the chart ofFigure5, all of these numbers would not be 7 necessary.

The selecting of-the proper blank for each *9 addition will be a simplematter. For. the .25 diopter addition, the blank selected would be thathaving #1 flint 'glass for the button and a base curve of threediopters. For

the -.50 diopter addition the grinder would 95 select .either a blankhaving. a #1 flint, glass and a six base curve, or one having'a #2 flintglass on a twobase curve. or a #3 on a three base curve, or a #4 on afour 1 base curve, according to which one of the a blank having #5 flintglass and a nine base curve. For the second possibility he would use a#6 flint glasson a nine base curve. For the third possibility he would 5use a #7 flint glass on a six base curve. In

every instance the basecurve selected would depend upon the curvature tobe ground on the rear face in order to give the desired power throughthe majorlens.

It will readilybe appreciated that for the corrections which areordinarily met with it is not necessary to use as many-as the sevendifferent ind'icesof flint glass." In

onlyexceptional cases do prescriptions call I for additions below .50diopter or above 4.00 diopters. For instance, as appears from Q thechart of Figure 6, prescriptions within that range can be taken care ofby a set of Y blanks the buttons of which are made from scope of myinvention as .only numbers 4, 5 and 6 0f the flint glass tabulated in Fiure 4. It will be Observed that these com inations could be satisfiedwith only seven numbers of blanks, as follows: a, #4 flint glass on athree base curve; a #4 flint glass on a six base curve; a #5 flint glasson a three base curve; a #5 flint glass on a six base curve; a #5 flintglass on a nine base curve; a #6 flint glass on a six base curve; and a#6 flint glass on a nine base curve. Y

\Vhile I have explained and illustrated my invention as adapted to planointerfaces between the two pieces of glass, it is evident that theprinciple of my invention is not limited to that particular geometricalform of surface, but that the surface as well as the indices ofrefraction of both the fl nt and crown glasses as above mentioned'may bemodified without departing from the pointed out in the appended claims.

I claim:

1. A product of manufacture comprising a set of bifocal lens blanksformed by fusing a small piece of glass having a relatively high indexof refraction to a major piece of glass of lower index of refraction,the set being made up of a plurality of series according to base curveand each series being made up of a plurality of units according to theindex of refraction of the small piece of glass, the index of thesmall'piece of glass being different for each'unit of a' series butbeing the same for the correspond ing units of the several series andthe base curvebeing different for the several series but being the samefor the units of each reassess 2. A product of manufacture comprising aset of bifocal lens blanks formed by fusing a small piece of glasshaving a relatively high indexof refraction to a major piece of glass oflower index of refraction the blanks all having substantially the samegeometrical surface between thetwo pieces of glass and the set beingmade up of a plurality of series according to base curve and eachhighindex of refraction to a major piece of glass of lower index ofrefraction, the

blanks all having substantially a plane surface between the two piecesof glass and the set being made up of a plurality of series according tobase curve and each series being made up of a plurality of unitsaccording to the index of refraction of the small piece of glass, theindex of the small piece i of glass being different for each unit of aseries but being the same for the corresponding units of the severalseries and the base curve being different for the several series butbeing the same for the units of each series.

In witness whereof, I hereunto subscribe my signature.

GEORGE A. CLEMENT.

